Pope Saint Nicholas I (Latin: Nicolaus I), also called Saint Nicholas the Great, was Pope from 24 April 858 to his death in 867. He is remembered as a consolidator of papal authority and power, exerting decisive influence upon the historical development of the papacy and its position among the Christian nations of Western Europe. Nicholas I asserted that the pope should have suzerain authority over all Christians, even royalty, in matters of faith and morals.

He is venerated as a saint in the Roman Catholic Church, with a feast day on 13 November.

He refused to grant an annulment to King Lothair II of Lotharingia from Teutberga so that Lothair could marry his mistress Waldrada. When a Council pronounced in favor of annulment, Nicholas I declared the Council to be deposed, its messengers excommunicated, and its decisions void. Despite pressure from the Carolingians, who laid siege to Rome, his decision held. During his reign, relations with the Byzantine Empire soured over his support for Ignatius, Patriarch of Constantinople, who had been removed from his post in favor of Photius.